Technical Abstract:
The amount of soil water lost during stage one and stage two soil water evaporation is of interest to crop water use modelers. The ratio of measured soil surface temperature (Ts) to air temperature (Ta) was tested as a signal for the transition in soil water evaporation from stage one to stage two drying. Hourly soil water evaporation (E) measurements were made in bare soil in weighing lysimeters (surface area of 0.75 m**2) containing monolithic soil cores of clay loam, silt loam, sandy loam or fine sand located in a rain shelter facility. Lysimeter soil surface temperature was measured by four infrared thermometers; air temperature was measured at 0.25 m above the soil surface; E was measured by a platform scale beneath the lysimeter. Grass reference evapotranspiration (ETo) was calculated from meteorological measurements made at a nearby (about 500 m) weather station. After an initial evaporation phase when cumulative E often exceeded cumulative ETo, there was periods of time during the daytime hours when both stage one and stage two E occurred alternately. The changes between evaporation stages were indicated by measurements of periods when hourly E=ETo (stage 1) followed by those when hourly E<ETo and Ts/Ta approached 1 (stage 2), resulting in a drop in cumulative E rate compared with the initial evaporation phase. When E<ETo and Ts/Ta>1 occurred consistently during the day and often during the night, E rates stabilized at about 1-3 mm d**1. Nighttime E also contributed to stage one E. The change from Ts/Ta<1 to Ts/Ta>1 during the day did indicate a change in evaporation stages.